JavaScript Arrow Functions Quiz

x => x;

Yes, the function is valid. It has a single parameter and simply returns the parameter. (Quite often, such a function is referred to as an identity function.) For more info, refer to JavaScript Arrow Functions.

() => true;

Yes, the function is valid. It has no parameters (hence the parentheses (())) and simply returns the Boolean true. For more info, refer to JavaScript Arrow Functions: Syntax of arrow functions.

_ => true;

Yes, the function is valid. It has a parameter called _ (remember that _ is valid in naming identifiers in JavaScript) and simply returns the Boolean true. For more info, refer to JavaScript Arrow Functions: Syntax of arrow functions.

() => { x: 10 };

No. The object literal here would be treated as the body of the arrow function, containing the code x: 10 in it. The correct way to represent the object literal is to encapsulate it inside a pair of parentheses (()). Hence, the correct choice is (B). For more info, refer to JavaScript Arrow Functions: Returning an object literal.

Absolutely true. In fact, by not creating a local arguments object, an arrow function actually encourages the use of rest parameters. For more info, refer to JavaScript Arrow Functions: No own arguments object.

var y = [1, 2, 3].map(x => x ** 2);

The mapping function maps each number of the given array to its square. Hence, [1, 2, 3] becomes [1, 4, 9], and this goes with choice (C). For more info, refer to JavaScript Arrow Functions: Examples.

var y = [1, 2, 3].map(x => { x ** 2 });

The mapping function maps each number of the given array to undefined, since there is no explicit return in the function's body (denoted via the curly braces ({})). Hecne, the correct choice is (D). For more info, refer to JavaScript Arrow Functions: Examples.

To 'close over this' simply means that an arrow function obtains its this from its outer lexical environment. It doesn't create a this binding of its own unlike a regular function in JavaScript. Hence, the correct choice is (B). For more info, refer to JavaScript Arrow Functions: Closing over this.

False. Arrow functions don't create their own local arguments object unlike regular functions in JavaScript, perhaps to encourage the much better way of using rest parameters. For more info, refer to JavaScript Arrow Functions: No own arguments object.

console.log(() => arguments);

The code actually throws an error as the arrow function returns an undefined variable arguments. Hence, the correct choice is (C). For more info, refer to JavaScript Arrow Functions: No own arguments object.

Arguments []

undefined

function f() {
   return () => {
      console.log(arguments);
   };
}

var f2 = f(10, 20, 30);
f2(1, 2, 3);

The call to f2(1, 2, 3) (in the last line) logs the arguments object. Since, the function f2() itself doesn't define arguments (as it's an arrow function), its value is taken from the outer environment where arguments holds the list [10, 20, 30]. Hence, the correct choice is (A). For more info, refer to JavaScript Arrow Functions: No own arguments object.

Arguments [10, 20, 30]

Arguments [1, 2, 3]